1. Field of the Invention
This invention relates to a high-refractivity plastic lens resin having a composition containing diallyl-based urethane and urea resin.
2. Description of the Related Art
Plastic lenses have been finding utility as eye lenses and camera lenses and in optical devices in recent years since they are lighter, less fragile and dyeable compared with inorganic lenses. As a resin employed widely for these applications these days, there is a resin obtained by subjecting diethylene glycol bisallylcarbonate (hereinafter called "CR-39") to radical polymerization. This resin has various merits, such as excellent impact resistance, light weight, superb dyeability, and good machanicability. Its refractivity is, however, small (n.sub.D =1.50) compared with inorganic lenses (n.sub.D =1.52). In order to achieve optical characteristics equivalent to glass lenses, it is essential to increase the central thickness, peripheral thickness and curvature of a lens, thereby unavoidably increasing the overall thickness of the lens. There is hence an outstanding demand for the development of a lens resin having a higher refractivity in order to overcome this inconvenience. As lens resins which can purportedly materialize higher refractivity, there have been proposed polycarbonate (n.sub.D =1.58 to 1.59) and polystyrene (n.sub.D =1.59 to 1.60), which are thermoplastic resins and which are therefor difficult to be processed into eyeglass lenses. The development of a lens resin which has high refractivity (n.sub.D &gt;1.6) and which can be processed by a cast-molding method, as CR-39 can be, is therefore desired. It was proposed that resins be obtained by a reaction between a polyisocyanate and polyols or mercapto group compound. However, these resins have gas bubbles formed therein during the polymerization process and are difficult to release from a mold. It is necessary to pretreat the glass or metal lens mold surface with a proper external mold releasing agent in order to form a release mold. However, the thickness of this release mold is difficult to control, and the precision degree of the lens surface is therefore difficult to maintain, resulting in the attachment of part of the lens resin to the release mold and damage on the surface of the lens. Alternatively, a proper internal mold releasing agent may be used for releasing purposes. The solubility of this internal mold releasing agent is small with respect to the mixture of polyisocyanate and polymercapto group compound, reducing the transparent degree of the lens resin.